GEOLOGICAL FEATURES OF ROCKINGHAM COUNTY.
JUSTUS H. CLINE, M.A.,
Assistant Geologist, Virginia State Geological Survey.
On the basis of both geological and physiographic features the state of Virginia is divided into three great provinces: the Coastal Plain on the east, the Piedmont Plateau in the central part, and the Appalachian Mountain province on the west. Rockingham County lies entirely within the latter province.
The Appalachian Mountain province is further divided into three sub-provinces, each of which shows more or less marked differences in the topographic types represented, the Old Appalachians, or Blue Ridge, on the east; the Great Valley, in the centre; and the New Appalachians, or Alleghany Ridges, on the west. Each of the three subdivisions is well represented within the borders of the county.
The topography of the county, in keeping with that of the Appalachian Mountain province in general, is varied and picturesque; and with the exception of the main part of the Blue Ridge may be defined as the parallel ridge and valley type; the ridges being generally parallel with each other and extending in a northeast and southwest direction, in keeping with the strike of the underlying rock-structure.
The Blue Ridge, which forms the eastern boundary of the county, is distinct topographically from the rest of the area in that it presents an uneven and knobby crest, and shows and absence of the parallel ridge type in its main part, which is so well developed both in the valley and the ridges to the west. The foothills, or lower ridges, which flank the Blue Ridge on its western slope, being made up of rocks similar both in origin and structure to those of the west, partake
more nearly of their topographic characters and really belong to the New Appalachian type. The highest point in the Blue Ridge in Rockingham County is High Knob, which attains and elevation of over 3600 feet above sea level.
The Shenandoah Valley province, which makes up the greater part of Rockingham County, when viewed from an elevation appears as a broad, undulating plain, traversed by a series of low ridges which barely appear above it. In these ridges the cherty beds of limestone, which are more resistant than the surrounding rocks, have given rise to a series of round, conical hills which dot the landscape at intervals of a few miles, and have an elevation of from two hundred to three hundred feet above the plain. Good examples of these are Round Hill near Bridgewater and Mole Hill near Dayton. The Shenandoah plain slopes gently toward the southwest as a rule and attains an elevation of about 1500 feet at Harrisonburg. The most conspicuous feature of the Valley province within the county is the Massanutten Mountain, which divides the northern part into two unequal divisions. This peculiar mountain, while only within a few miles of the Blue Ridge and parallel with it, shows no geological kinship with it, for it is in reality an outlier of the New Appalachian ridges about eighteen miles to the west. The mountain ends abruptly in a fine peak which is locally known as Peaked Mountain, 2900 feet above the sea. Laird’s Knob, a few miles northward, attains an elevation of more than 3400 feet. The topography of the Massanutten Mountain is identical with that of the Alleghany ridges.
The Alleghany ridges and valleys embrace the western third of the county. These ridges show a more or less even crestline, and are arranged with a decided parallelism. The most elevated part of the county is within this province. High Knob in the Shenandoah Mountain has an elevation slightly more than 4200 feet. Practically all types of ridges found in an eroded region of folded sedimentary rocks occur here: the monoclinal, anticlinal, and synclinal. These ridges are frequently cut by gaps through which streams are now
flowing or have flowed in a former period. The valleys between the ridges are narrow.
The drainage of Rockingham county belongs entirely to the Shenandoah system. The two master streams are the North Fork, which flows west of he Massanutten Mountain, and the main part of the Shenandoah, which flows through the Page valley east of the same mountain. These streams and their tributaries have cut the Shenandoah plain into a deep intaglio, and they now flow in channels from 100 to 300 feet below it. The streams of the county have for the most part adjusted themselves to the underlying structural conditions, and consequently they tend to flow in a northeasterly direction. The chief exception to this is the North River, which flows eastward near the southern border of the county. This stream, instead of flowing parallel with the strike of the rock formations, takes a course at right angles to the strike and maintains it till its influx with the main Shenandoah near the foot of the Blue Ridge. This is explained by the fact that its course antedates the folding of the rock strata which has occurred in the Appalachian province. The county is exceedingly well watered, and springs of large size are frequent.
The rocks of the county fall under the two great lithologic types, igneous and sedimentary. Both types occur in all three of the sub-provinces. It is understood that igneous rocks are those which have solidified from a molten condition, while the sedimentaries are those which are composed of material derived from the waste of land and redeposited in the form of mud, sand, gravel, chemical precipitants, etc., chiefly in lakes and seas.
The only dominantly igneous area in the county is the main part of the Blue Ridge. The formations of the rest of the county are sedimentary, with very minor exceptions. Certainly less than one per cent. Of the rocks of the county which outcrop at the surface are of igneous origin, and all these with the few noted exceptions are entirely in the Blue Ridge.
The igneous rocks of the Blue Ridge in Rockingham County are of the basaltic type, which represent material
thrown out on the surface by the extrusive action of ancient volcanoes. The material in its original form was dark or almost black in color, but subsequent alterations and metamorphic changes have usually converted it into rock varying in color from a dark bluish-green to a light green, dependent on the secondary mineralogical constituents. Where these minerals are secondary amphibole and chlorite the rock is bluish green in color; and where epidote is the dominant alteration product the color is light green. There are all gradations between these different colors depending on the proportions of these minerals. Originally the essential minerals which made up the basalt was plagioclase feldspar, magnetite, and pyroxene, but subsequent alterations have almost entirely broken down the original mineral components giving rise chiefly to the three above mentioned, so that at present the rock may properly be termed an epidosite where the dominant mineral is epidote, an amphibolite where the dominant mineral is amphibole, and a chlorite-schist where the dominant mineral is chlorite. For all practical purposes the two latter types may rightly be thrown together and termed amphibolite-chlorite schists. The general name for this basaltic formation wherever it occurs in the Blue Ridge and the Piedmont plateau is Catoctin Schist, so called because it usually presents a slaty or schistose structure, induced by the folding of the earth’s crust and the development of the secondary minerals contingent upon such conditions. The rock frequently shows material picked up by the liquid lava streams as they flowed over the ancient land surface covered with the material of older flows; and gas bubbles, or amugdules, are common in the upper surfaces. These cavities have since been filled by percolating waters carrying mineral matter in solution, the resulting minerals being usually epidote, calcite, and quartz. It is within the Catoctin Schist that the copper deposits of the Blue Ridge occur. The rock has been assigned to the Algonkian Period and is therefore one of the oldest formations occurring in the state, and certainly the oldest in the county.
Late investigations have revealed that igneous rocks also occur in the county within the Valley province, in limited quantity, in the form of dikes. These rocks outcrop in the vicinity of Cross Keys and at intervals entirely across the county; the most westerly outcrop reported being in the vicinity of Brock’s Gap. A characteristic exposure of these rocks is found near Harrisonburg, where the road leading from Harrisonburg to Keezletown crosses the Chesapeake-Western Railway tracks. The rock is of dark color and medium to fine grained. It weathers to rounded boulders, which are broken only with great difficulty. It is a typical diabase and is composed of the minerals magnetite, augite, olivine, and plagioclase feldspar as essential ingredients. The rock is comparatively fresh, and is probably the youngest of the rock formations in the county, having been forced while in a molten condition into the fissures of the limestone and shale formations of the mountains. While the exact age of the rock cannot be determined, on lithological grounds it may be assigned to the Triassic. (1) The rock, owing to its high lime and iron content, makes road material of the first quality, since it has the necessary ingredients to bind and is far superior to limestone in lasting qualities.
The sedimentary rocks of the county present every phase of sedimentary
(1) Thos. L. Watson and Justus H. Cline, “Dikes of the Shenandoah Valley,” in preparation.
possibilities, and rocks of every age occur from the beginning of the Cambrian to the Lower Coal period. Starting with the oldest rocks of this type in the county we find sandstones, which are frequently conglomeritic, and shales of Cambrian age making up the foothills or lower ridges which flank the Blue Ridge on it s western slope. This formation rests on the Catoctin Schist as a base and the material of which they are composed was probably derived from it and from other igneous and sedimentary formations to the east of the Blue Ridge. The thickness of the Cambrian sandstone and shale formation may be estimated at from 2000 to 3000 feet. The lower part of the formation is dominantly sandstone, and the upper part shaley. The age of the formation has been determined by the remains of animal life which are preserved extensively in one of the sandstone beds in the form of worm borings by the species scolithus linearis, characteristic of Cambrian.
The Cambrian shales grade into Shenandoah limestone, which succeeds them without break in the sedimentary record, the lower part of the limestone formation also being Cambrian.
The thickness of the Shenandoah limestone is from 1500 to 2500 feet. The upper part is definitely known to be Ordovician in age, from the fossils it contains, but there is no definite line of division marked between the known Ordovician and the known Cambrian. Five distinct divisions of this limestone in central western Virginia have been described by Prof. H. D. Campbell. These are, in ascending order, (1) Sherwood Limestone of Lower Cambrian age, (2) Buena Vista shales of Lower or Middle Cambrian age, (3) Natural Bridge limestone of Middle and Upper Cambrian and Lower Ordovician, (4) Murat limestone, and (5) Liberty Hall limestone of Middle Ordovician age. The Murat and Liberty are probably absent in Rockingham County, and their places occupied by the above and dolomitic limestones of Stones River age and the pure and argillaceous strata designated as the Chambersburg formation. Fossils found near Harrisonburg have identified both these formations at that place.
The Shenandoah limestone varies in color from a grey to dove color and blue. It contains frequent cherty beds, the weathering of which gives rise to the ridges and gravelly hills common in the county. Shaley beds also occur within the limestone of varying thickness. The most prominent of these are the Buena Vista shales of Cambrian age, which are easily recognized by their reddish color. In composition the rock varies from an almost pure limestone to dolomite, in which the lime is replaced largely by magnesia. The rock also shows widely varying percentages of clayey material and silica. The Chambersburg and Stones River formations and the associated Trenton shales afford splendid material for the manufacture of Portland cement.
In the upper part of the Shenandoah limestone shaley beds appear, which increase in frequency and thickness till the formation entirely gives way to the Martinsburg shale. This shale formation occupies a prominent synclinal trough extending entirely across the county. This syncline is known as the Massanutten syncline, since its position is partly occupied by the mountain of the same name. The syncline is very persistent, and extends far beyond the limits of the mountain both to the northeast and southwest. There are also other minor areas of the shale formation which may be seen on the accompanying map. The thickness of the formation is about 2000 feet. The fine blue slate lands of the county are underlain by the lower beds of this formation. The weathering of the shale gives rise to the characteristic rounded hills with oval crests, often very similar in form to the drumlins of glacial origin in the northern part of the United States. The contact between the shale and limestone can readily be detected by the abrupt change in the topography.
The Martinsburg shale is succeeded by the Massanutten sandstone, so called from its occurrence in the mountain by that name. It is to the resistant character of this rock that the Massanutten mountain owes its existence. The thickness of the sandstone is about 500 feet. It varies from a reddish sandy shale to a coarse conglomerate and light grey massive
bedded quartzite. A typical exposure of this quartzite is found at the nose of Peaked Mountain. The formation also occurs in the Alleghany ridges. Its age is Silurian.
The rocks of the Alleghany ridges are as follows, given in order of age: (1) the Martinsburg shale and (2) Massanutten sandstone, which have been described above; (3) Rockwood formation of Silurian age, composed of reddish micaceous sandstones, which grade upward into reddish and brown shales capped with a bed of grayish to yellowish quartzite. The thickness of the formation is about 200 feet. (4) Lewistown limestone of Silurian age, containing numerous fossil corals and braciopods and remains of sponges and microscopic organisms. Its thickness is about 100 feet. (5) Monterey sandstone of Silurian and Devonian age, in part calcareous. The thickness is about 300 feet. (6) The dark colored Romney shales of Devonian age, which were deposited on the eroded surface of the Monterey sandstone. This non-conformity represents the only break in the sedimentary cycle within the bounds of Rockingham County. The thickness of the formation approaches 1000 feet. (7) Jennings formation, also of Devonian age, which is made up of olive to buff colored shales interstratified with massive fine-grained sandstones. Its thickness is about 3000 feet. It is succeeded by the (8) Hampshire formation, made up of thinly bedded grey and reddish sandstone and thick bedded sandstone, all interbedded with thin layers of shale. The formation is as thick as 1400 feet, and it is of Devonian age. (9) The Pocono sandstone of Mississippian age, which is the youngest of the sedimentaries in the county. It is composed of light grey sandstones of a rather coarse texture, which are interbedded with thin layers of semi-anthracite coal. It is about 700 feet in thickness.
The structure of the rocks of Rockingham County can best be understood by reference to the accompanying structure section. It will be remembered that the sedimentary rocks of the county were originally laid down by water in the order in which they now occur, but in a horizontal position. The
beds are rarely found in this position, but have been folded into anticlines and synclines and frequently broken by great overthrust faults and also by simple gravity faults.
Since the folding and faulting of the region, erosion has been active, so that now the mountain ridges frequently occupy the position of the synclines, as in the case of the Massanutten mountain; and the river valleys in the same way occupy the position of the anticlines. The Valley province is not a structural valley, but it is entirely the product of erosion. The material which once occupied its position, being less resistant to the forces of degradation, was disintegrated by chemical and mechanical forces and carried by the streams to the sea. The streams which have been responsible for this work have suffered likewise many changes, and now in a small measure only resemble their early ancestors. The drainage of the county at the beginning of the long cycle of erosion which developed its present land forms seems to have been controlled by tow master streams. One of these streams occupied a position similar to North River, in the latitude of Bridgewater, and the other a position similar to the North Fork, in the latitude of Brock’s Gap; the North River flowing across the Valley and Blue Ridge, possibly through Brown’s Gap, and the North Fork across the Valley and the Massanutten Mountain at New Market and the Blue Ridge opposite Luray. Since the limestones of the Valley were more easily eroded than the harder rocks of the Blue Ridge, and since the Potomac came to be the master stream because of its size, tributaries of the Potomac flowing northward over the soft rocks of the valley were finally able to intercept these streams, first the North Fork at Luray and later the North River at Port Republic. Subsequently to these captures the Valley has been lowered many hundred feet below its level at the time the captures took place.
The limestones of the Valley province are responsible for the numerous beautiful limestone caverns and bold springs which are so common, as well as for the remarkable fertility of the soils of the county, which has made her one of the
most desirable agricultural districts in the entire country. The hand of Providence working through long ages has prepared a habitation for men, which in the beauty of topography and landscape, fertility of soil, excellence of water, delightfulness of climate, luxuriance of vegetation, and all natural environment that makes for human happiness, can hardly be excelled in the entire world.
By Miss Ruth Conn.
Where the peak of old Massanutten
Doth bare his broad dome to the skies,
And clad with the strength of Creation
Unmindful of ages doth rise,
He guards day and night our green valley;
For Nature who made it so fair,
Grew alarmed for her beautiful treasure,
And placed him as sentinel there.
When the gray morning mists of the Valley,
That are wont to encircle his crest,
Have long faded into the sunlight,
And wandering winds are at rest,
When from off of his summit has faded
The glow of the evening bars,
He brings from the worlds shining o’er him
Sweet dreams to our “Child of the Stars.”
This sacred trust of Creation
He kept since the world began,
Till he smiled on the red man’s wigwam
And the hut of the first white man.
And oft in the struggles that followed,
He echoed the martial tramp,
And sheltered the fires where our fathers
Lay waiting with Stonewall in camp.
He has stood with us in every struggle,
Though burdened methinks with our pain;
He has pointed to courage and patience,
And helped us new visions to gain.
Oh, fairer than Italy’s mountains,
Or Switzerland’s snow-crowned towers,
He is to the sons of the Valley –
This rugged old mountain of ours!
Dear old Peak, thou art guarding thy treasure:
May men to their trust prove as true!
Not one of Virginia’s blue mountains
Is so dear to our hearts as are you.
McGaheysville, Rockingham County,
Virginia, May, 1912.